February 8, 2022 Do Young Trees Suck Up More Carbon?

Which is better at slowing climate change: mature forests or young ones? It’s a mostly settled debate among climate scientists who have dedicated their careers to studying how trees suck carbon from the air and store it in their trunks, branches and even in the soil beneath their roots.

But that debate is being rekindled in Washington state, as officials weigh public comments that could ultimately determine how the state will implement its new cap and trade program—set to begin in January 2023—as part of a bigger statewide push to reduce greenhouse gas emissions in the coming decades. Climate activists are pushing back against a comment left by a trade group representing the timber industry, which suggests that cutting down old trees and replacing them with young ones is better for the climate than leaving the forests untouched.

“Offset protocols should incentivize active forest management, which is more effective in capturing and storing atmospheric carbon in forest and wood product carbon pools than a policy of hands-off management that precludes periodic harvests and the use of wood products,” the American Forest Resource Council wrote to Washington regulators in a Jan. 26 comment.

In its reasoning, the trade group cites a 2016 study, part of a body of research that has found that younger forests absorb carbon from the air at a faster rate than older ones. As trees age, the studies suggest, their ability to pull in carbon slows down until eventually the trees die and release that carbon back into the atmosphere as the wood decays or is burned. Therefore, the American Forest Resource Council argues, Washington regulators should be prioritizing planting young trees over protecting old ones.

But some of the nation’s top forestry and climate experts say that that conclusion is out of touch with reality, and that what matters more is the total amount of carbon those forests keep out of the atmosphere, not the rate at which they absorb carbon.

Old-growth forests, the kinds that have been around for decades and even centuries, store significantly more total carbon than young forests, with the largest trees playing the biggest role. Alaska’s Tongass National Forest, for example, stores the equivalent of about 8 percent of the carbon stored in all the forests of the lower 48 states combined. Many of the trees in the Tongass are over 800 years old.

But once a forest gets harvested, chances are that the carbon stored in those trees will soon return to the atmosphere, said Beverly Law, a professor emeritus at Oregon State University and a leading voice in the intersecting fields of forest ecosystems and carbon sequestration. 

In 2019, Law co-authored a study that found that 65 percent of the trees harvested in Oregon over the past 115 years have returned to the atmosphere in the form of carbon dioxide, while 16 percent ended up in landfills. Those trees often became short-lived consumer products such as paper or cardboard.

Conversely, only 19 percent of those trees harvested in Oregon turned into longer-term consumer products—such as the wood used for building homes—that continue to store carbon today, the research found.

“We’ve looked at these numbers over time, and cumulative amounts, it goes to the atmosphere fairly quickly,” Law said, referring to the carbon held in harvested trees. “Most of it ends up in the atmosphere within a few decades.”

Research has also shown that young trees are more susceptible than older trees to dying from heat waves, drought and wildfires, all of which are worsening under climate change. And newly planted forests can take decades to store as much carbon as the older forests they’re replacing, Law said.

All things considered, Law said, states should be prioritizing the protection of their old-growth forests, not replacing them with young trees as the timber industry suggests, as they pursue tough plans to reduce emissions and stave off the worst of the climate crisis. “We only have the next 10 to 30 years—and we have to start now—to get our emissions down,” she said. “That is not getting our emissions down. That’s taking us back to ground zero and trying to regrow again.”

That’s it for this issue of Today’s Climate. I’ll be back in your inbox on Friday.

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